Generally, an ion source is a device that ionizes gas molecules and focuses, accelerates, and emits the ionized gas molecules and/or atoms in a beam for a variety of technical and industrial applications. For example, ion sources may be used as thrusters on space craft. Ion sources are also used in semiconductor material and device processing, optical filter processing, and metrology, among other applications. Common uses of ion sources include without limitation cleaning, assisting deposition (by chemically or physically activating), polishing, etching and/or depositing of thin-film coatings. Typically, a substrate is passed through an ion beam (e.g., an etching beam) for such processing.
An anode layer source (ALS) typically refers to a Hall-current type ion source having a grounded cathode and a DC-biased anode. The working gas is fed into an ionization region in the vicinity of the anode and the cathode, where the combination of electric and magnetic fields in this region ionizes the molecules of the working gas and accelerates each ion away from the ionization region toward a target. The ionization region generally forms a closed-loop (e.g., a race track shape) in the face of the ion source. The shape of this closed-loop “race track” may be round, oval, linear with rounded ends, or many other closed shapes.
One benefit to an ALS is that an ALS does not require a hot cathode electron source (e.g., filament cathode, hollow cathode, or RF neutralizer) with a separate power supply to sustain the plasma. ALS cathodes are passive, cold cathodes, typically made of steel. The cathodes also function as pole pieces for the ALS magnetic circuit. The cold cathodes do not actively emit electrons, but ions bombarding the cathodes release secondary electrons that help to sustain the discharge.
One problem with an ALS, however, is that the ions striking the cathodes can also sputter material from the cathodes. The sputtered cathode material may enter the process as a contaminant. Such cathodes are typically steel or magnetic stainless steel, so the primary contaminant is iron, although other contaminants may also exist. The sputtered material tends to emit across a wide range of angles. As a result, the sputtered material tends to impinge the substrate surface outside the envelope of the etching beam as well as inside the envelope of the etching beam. Depending on the type of ion source, the operating regime, and the application, there may be other ion source electrodes or adjacent components that also sputter in a similar matter and contribute to substrate contamination.
Most contaminants impinging the substrate surface prior to and during the passing of the substrate through the etching beam are etched away by the beam. However, the contaminants that impinge the surface of the substrate after the substrate has passed through the etching beam remain as contaminants. In other words, a substrate tends to acquire a new layer of contaminants after exiting the envelope of the ion beam. Therefore, for example, etching a substrate using an ALS may yield an etched substrate having an unacceptable concentration of iron contaminants sputtered from the ALS itself.